- Multimedia：video, speech and music
- non-linear ⇒ linear
直接求解（较困难）⇒ iteration methods
An×nx=b ：n 足够大时，x=A∖b 将会变得是分困难，此时可以使用子空间迭代的方式，Kxi+1=Kxi+b−Axi
其中的r0=b−Ax0 记为 remainer；
- 有正有负，进行统一处理（比如衡量误差）时，一般首先执行平方操作，以避免加和时的相互抵消；
- 方差；
- MSE；（均方误差）
1. 凸函数 & 非凸函数
- Convex functions are attractive because they can be more reliably minimized than non-convex functions.
- However, non-convex penalty functions can lead to enhanced sparsity of sparse signals.
2. dimensionality reduction
- Ideally, the reduced representation should have a dimensionality that corresponds to the intrinsic dimensionality of the data；
3. explanatory
- Real-world data, such as :
- speech signals,
- digital photographs,
- fMRI scans
4. 数据的来源（data acquisition ）
- 传感器：RFID, robot & cameras